The middle ear is a sterile, air-filled cavity separated from the outer ear by the eardrum. Attached to the eardrum are three ear bones that vibrate when sound waves strike the eardrum. Vibrations are transmitted to the inner ear, which generates nerve impulses that are sent to the brain. Air may enter the middle ear through the Eustachian tube, which opens in the walls of the nasopharynx.
The nasopharynx is located posterior to the nasal cavities. The nasopharynx is lined by the respiratory epithelium and stratified squamous epithelium. Beneath the respiratory epithelium, the abundant mucosa-associated lymphoid tissue (MALT) forms the nasopharyngeal tonsil (adenoids).
Bacterial infection or inflammation of the middle ear is mainly observed in children. Due to the isolation of the middle ear, it is suggested that development of middle ear infections requires the involvement of the nasopharynx and Eustachian tube. Infections with Streptococcus pneumoniae (S. pneumoniae) are one of the major causes of middle ear infections, as well as bacteremia, meningitis, and fatal pneumonia worldwide (Butler, J. C., et al., American Journal of Medicine, 1999, 107:69S–76S). The rapid emergence of multi-drug resistant pneumococcal strains throughout the world has led to increased emphasis on prevention of pneumococcal infections by vaccination (Goldstein and Garau, Lancet, 1997, 350:233–4).
Protein antigens of S. pneumoniae have been evaluated for protective efficacy in animal models of pneumococcal infection. Some of the most commonly studied vaccine candidates include the the PspA proteins, PsaA lipoprotein, and the CbpA protein. Numerous studies have shown that PspA protein is a virulence factor (Crain, M. J., et al., Infect Immun, 1990, 58:3293–9; McDaniel, L. S., et al., J Exp Med,1984, 160:386–97), but is antigenically variable among pneumococcal strains. Additionally, a recent study has indicated that some antigenically conserved regions of a recombinant PspA variant may elicit cross-reactive antibodies in human adults (Nabors, G. S., et al., Vaccine, 2000, 18:1743–1754). PsaA, a 37 kDa lipoprotein with similarity to other Gram-positive adhesins, is involved in manganese transport in pneumococci (Dintilhac, A., et al., Molecular Microbiology, 1997, 25(4):727–739; Sampson, J. S., et al., Infect Immun, 1994, 62:319–24.) and has been shown to be protective in mouse models of systemic disease (Talkington, D. F., et al., Microb Pathog, 1996. 21:17–22). The surface exposed choline binding protein, CbpA, is antigenically conserved and also is protective in mouse models of pneurnococcal disease (Rosenow, C., et al. Molecular Microbiology, 1997, 25:819–29). Since nasopharyngeal colonization is a prerequisite for otic disease, intranasal immunization of mice with pneumococcal proteins and appropriate mucosal adjuvants has been used to enhance the mucosal antibody response and thus, the effectiveness of protein vaccine candidates (Briles, D. E., et al., Infect Immun, 2000, 68:796–800; Yamamoto, M., et al., A. J Immunol, 1998, 161:4115–21).
The currently available 23-valent pneumococcal capsular polysaccharide vaccine is not effective in children of less than 2 years of age or in immunocompromised patients, two of the major populations at risk from pneumococcal infection (Douglas, R. M., et al., Journal of Infectious Diseases, 1983, 148:131–137). A 7-valent pneumococcal polysaccharide-protein conjugate vaccine, was shown to be highly effective in infants and children against systemic pneumococcal disease caused by the vaccine serotypes and against cross-reactive capsular serotypes (Shinefield and Black, Pediatr Infect Dis J, 2000, 19:394–7). The seven capsular types cover greater than 80% of the disease isolates in the United States, but only 57–60% of disease isolates in other areas of the world (Hausdorff, W. P., et al., Clinical Infectious Diseases, 2000, 30:100–21). Therefore, there is an immediate need for a vaccine to cover most or all of the disease causing serotypes of pneumococci.
Iron is an essential element for colonization and infection by many pathogenic bacteria. Prevention of the acquisition process should result in a reduction of colonization and a lower disease potential. Iron acquisition complexes in successful pathogens such as, but not limited to, N. gonorrheae, N. meningitidis, M. catarrhalis, and H. influenzae have been evaluated for their vaccine potential by other laboratories (Conte, M. P, et al., Infection and Immunity, 1999, 64:3925; Gray-Owens, S. D., et al. Infection and Immunity, 1995, 64:1201; Luke N. R. et al., Infection and Immunity, 1999, 67:681; Pettersson, A, et al., Infection and Immunity, 1993, 61:4724). Thus, isolation of the structures responsible for iron acquisition could lead to vaccine candidates.